The accurate measurement of turbidity has been an urgent requirement in a number of diverse analytical, engineering and manufacturing settings. For example, applications for turbidity sensor technology range from the manufacturing of pharmaceuticals and medical devices, to water purification, environmental monitoring and the control of dishwashing and clothes washing machine operation. In its simplest form, a turbidity sensor consists of a light emitter separated from a light intensity sensor by a volume or flow of a sample to be analysed. As the turbidity of a sample increases, the intensity of the light transmitted directly along the optical path and falling on the light sensor decreases (thus giving a crude measure of the turbidity of the sample). Although adequate for many applications, this basic form of turbidity sensor cannot distinguish between a decrease in the received signal caused by absorption by the intervening medium or suspended particles, by the effect of actual scattering caused by suspended particulate matter along the optical path, or by systematic variability caused by fluctuations in emitter or detector performance (i.e., variability caused by emitter and sensor temperature fluctuations).
A more sophisticated, accurate and common form of turbidity sensor consists of a single light emitter positioned inline with a light detector on a common optical axis, and a secondary light detector positioned at 90° relative to the optical axis of the first emitter detector pair. In this off axis position, the second light detector's input is proportional to the amount of the emitter's output which has been scattered or reflected perpendicular to the original optical axis. When analysed in conjunction with the output of the on-axis detector's output, a ratiometric estimate of turbidity or scattering can be achieved which is relatively immune to systematic measurement errors, or errors introduced by absorption by the suspended particles or sample medium. It is important to note, however, that under extreme turbid conditions the amount of the scattered signal able to reach the off axis detector can decrease to such an extent that the ratiometric output becomes meaningless. In addition, the requirement of a secondary off axis detector limits present turbidity sensors to the detection and quantification of turbidity in fluid mediums, or small samples of solid material.